From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: torikoshia <torikoshia@oss.nttdata.com>
Cc: Ian Lawrence Barwick <barwick@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: adding wait_start column to pg_locks
Date: Mon, 15 Feb 2021 15:17:51 +0900
Message-ID: <655f1a5b-4fa8-53ba-c3e2-8cee31587d37@oss.nttdata.com> (raw)
In-Reply-To: <337d5311-6113-15e0-bd85-90b47b55f5ae@oss.nttdata.com>
References: <a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com>
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<3db333a9-31f7-a225-8038-eeb835ba8f37@oss.nttdata.com>
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<40dfaa75-1058-e811-1f7c-4cf7203a3068@oss.nttdata.com>
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<067d5597e5de82fe47e5716991ab65c4@oss.nttdata.com>
<337d5311-6113-15e0-bd85-90b47b55f5ae@oss.nttdata.com>
On 2021/02/10 10:43, Fujii Masao wrote:
>
>
> On 2021/02/09 23:31, torikoshia wrote:
>> On 2021-02-09 22:54, Fujii Masao wrote:
>>> On 2021/02/09 19:11, Fujii Masao wrote:
>>>>
>>>>
>>>> On 2021/02/09 18:13, Fujii Masao wrote:
>>>>>
>>>>>
>>>>> On 2021/02/09 17:48, torikoshia wrote:
>>>>>> On 2021-02-05 18:49, Fujii Masao wrote:
>>>>>>> On 2021/02/05 0:03, torikoshia wrote:
>>>>>>>> On 2021-02-03 11:23, Fujii Masao wrote:
>>>>>>>>>> 64-bit fetches are not atomic on some platforms. So spinlock is necessary when updating "waitStart" without holding the partition lock? Also GetLockStatusData() needs spinlock when reading "waitStart"?
>>>>>>>>>
>>>>>>>>> Also it might be worth thinking to use 64-bit atomic operations like
>>>>>>>>> pg_atomic_read_u64(), for that.
>>>>>>>>
>>>>>>>> Thanks for your suggestion and advice!
>>>>>>>>
>>>>>>>> In the attached patch I used pg_atomic_read_u64() and pg_atomic_write_u64().
>>>>>>>>
>>>>>>>> waitStart is TimestampTz i.e., int64, but it seems pg_atomic_read_xxx and pg_atomic_write_xxx only supports unsigned int, so I cast the type.
>>>>>>>>
>>>>>>>> I may be using these functions not correctly, so if something is wrong, I would appreciate any comments.
>>>>>>>>
>>>>>>>>
>>>>>>>> About the documentation, since your suggestion seems better than v6, I used it as is.
>>>>>>>
>>>>>>> Thanks for updating the patch!
>>>>>>>
>>>>>>> + if (pg_atomic_read_u64(&MyProc->waitStart) == 0)
>>>>>>> + pg_atomic_write_u64(&MyProc->waitStart,
>>>>>>> + pg_atomic_read_u64((pg_atomic_uint64 *) &now));
>>>>>>>
>>>>>>> pg_atomic_read_u64() is really necessary? I think that
>>>>>>> "pg_atomic_write_u64(&MyProc->waitStart, now)" is enough.
>>>>>>>
>>>>>>> + deadlockStart = get_timeout_start_time(DEADLOCK_TIMEOUT);
>>>>>>> + pg_atomic_write_u64(&MyProc->waitStart,
>>>>>>> + pg_atomic_read_u64((pg_atomic_uint64 *) &deadlockStart));
>>>>>>>
>>>>>>> Same as above.
>>>>>>>
>>>>>>> + /*
>>>>>>> + * Record waitStart reusing the deadlock timeout timer.
>>>>>>> + *
>>>>>>> + * It would be ideal this can be synchronously done with updating
>>>>>>> + * lock information. Howerver, since it gives performance impacts
>>>>>>> + * to hold partitionLock longer time, we do it here asynchronously.
>>>>>>> + */
>>>>>>>
>>>>>>> IMO it's better to comment why we reuse the deadlock timeout timer.
>>>>>>>
>>>>>>> proc->waitStatus = waitStatus;
>>>>>>> + pg_atomic_init_u64(&MyProc->waitStart, 0);
>>>>>>>
>>>>>>> pg_atomic_write_u64() should be used instead? Because waitStart can be
>>>>>>> accessed concurrently there.
>>>>>>>
>>>>>>> I updated the patch and addressed the above review comments. Patch attached.
>>>>>>> Barring any objection, I will commit this version.
>>>>>>
>>>>>> Thanks for modifying the patch!
>>>>>> I agree with your comments.
>>>>>>
>>>>>> BTW, I ran pgbench several times before and after applying
>>>>>> this patch.
>>>>>>
>>>>>> The environment is virtual machine(CentOS 8), so this is
>>>>>> just for reference, but there were no significant difference
>>>>>> in latency or tps(both are below 1%).
>>>>>
>>>>> Thanks for the test! I pushed the patch.
>>>>
>>>> But I reverted the patch because buildfarm members rorqual and
>>>> prion don't like the patch. I'm trying to investigate the cause
>>>> of this failures.
>>>>
>>>> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=rorqual&dt=2021-02-09%2009%3A20%3A10
>>>
>>> - relation | locktype | mode
>>> ------------------+----------+---------------------
>>> - test_prepared_1 | relation | RowExclusiveLock
>>> - test_prepared_1 | relation | AccessExclusiveLock
>>> -(2 rows)
>>> -
>>> +ERROR: invalid spinlock number: 0
>>>
>>> "rorqual" reported that the above error happened in the server built with
>>> --disable-atomics --disable-spinlocks when reading pg_locks after
>>> the transaction was prepared. The cause of this issue is that "waitStart"
>>> atomic variable in the dummy proc created at the end of prepare transaction
>>> was not initialized. I updated the patch so that pg_atomic_init_u64() is
>>> called for the "waitStart" in the dummy proc for prepared transaction.
>>> Patch attached. I confirmed that the patched server built with
>>> --disable-atomics --disable-spinlocks passed all the regression tests.
>>
>> Thanks for fixing the bug, I also tested v9.patch configured with
>> --disable-atomics --disable-spinlocks on my environment and confirmed
>> that all tests have passed.
>
> Thanks for the test!
>
> I found another bug in the patch. InitProcess() initializes "waitStart",
> but previously InitAuxiliaryProcess() did not. This could cause "invalid
> spinlock number" error when reading pg_locks in the standby server.
> I fixed that. Attached is the updated version of the patch.
I pushed this version. Thanks!
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
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To: pgsql-hackers@postgresql.org
Cc: masao.fujii@oss.nttdata.com, torikoshia@oss.nttdata.com, barwick@gmail.com, robertmhaas@gmail.com, pryzby@telsasoft.com
Subject: Re: adding wait_start column to pg_locks
In-Reply-To: <655f1a5b-4fa8-53ba-c3e2-8cee31587d37@oss.nttdata.com>
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